An integrated circuit (IC) is a device consisting of a number of connected circuit elements, such as transistors and resistors, fabricated on a single chip of silicon crystal or other semiconductor material. During operation, an IC burns power causing the temperature of the IC to increase. An overheated IC can potentially result in reduced performance and even failure.
ICs, however, are typically packaged in such a way that it is difficult to directly measure the temperature at the active part of the die using a thermocouple or other external measuring device. As a result, the standard method for measuring the die temperature of an IC is to incorporate a thermal diode with known thermal characteristics into the design of the IC.
Thermal diodes are typically the base-emitter junction of a substrate connected PNP transistor. This junction may be modeled using the ideal diode equation. The equation of an ideal diode is:                     i        =                              I            s                    ⁡                      (                                          ⅇ                                  vq                  nkT                                            -              1                        )                                              (                  equation          ⁢                                          ⁢          1                )            where i is the forward biased current through the diode, Is is the saturation current of the diode, v is the voltage drop across the diode, q is the charge of an electron, n is an ideality factor, k is Boltzmann's Constant, and T is the temperature in Kelvin. The ideality factor n is constant for a given process technology. A range for this number is typically available in its product data sheet.
To measure the die temperature using a diode, the terminals of the diode are coupled to IC terminals. By applying a current i to the diode, the voltage drop v across the diode is measured at the terminals. For this measurement, the IC may or may not be running. By measuring the voltage drop at two different currents, i1 and i2, at constant temperature T, the value Is is cancelled out as shown in the equation:                                           i            1                                i            2                          =                                                            I                s                            ⁡                              (                                                      ⅇ                                                                                            v                          1                                                ⁢                        q                                            knT                                                        -                  1                                )                                                                    I                s                            ⁡                              (                                                      ⅇ                                                                                            v                          2                                                ⁢                        q                                            knT                                                        -                  1                                )                                              =                                                                      ⅇ                                                                                    v                        1                                            ⁢                      q                                        knT                                                  -                1                                                              ⅇ                                                                                    v                        2                                            ⁢                      q                                        knT                                                  -                1                                      .                                              (                  equation          ⁢                                          ⁢          2                )            
Equation 2 may be simplified by removing the 1's since they are negligible. Thus, the equation becomes:                                           i            1                                i            2                          =                                            ⅇ                                                                    v                    1                                    ⁢                  q                                knT                                                    ⅇ                                                                    v                    2                                    ⁢                  q                                knT                                              .                                    (                  equation          ⁢                                          ⁢          3                )            Given that the ratio of the currents is constant, the temperature is directly proportional to the difference in the two measured voltage drops, v1 and v2:       ln    ⁡          (                        i          1                          i          2                    )        =            ln      ⁡              (                              ⅇ                                                            v                  1                                ⁢                q                            knT                                            ⅇ                                                            v                  2                                ⁢                q                            knT                                      )              =                            ln          ⁡                      (                          ⅇ                                                                    v                    1                                    ⁢                  q                                knT                                      )                          -                  ln          ⁡                      (                          ⅇ                                                                    v                    2                                    ⁢                  q                                knT                                      )                              =                                                  v              1                        ⁢            q                    knT                -                                            v              2                        ⁢            q                    knT                    Solving for T:                     T        =                              (                          q                              kn                ⁢                                                                  ⁢                                  ln                  ⁡                                      (                                                                  i                        1                                                                    i                        2                                                              )                                                                        )                    ⁢                                    (                                                v                  1                                -                                  v                  2                                            )                        .                                              (                  equation          ⁢                                          ⁢          4                )            
Equation 4, however, does not include the effective series resistance, Rs, of the diode. Typically, long traces are a primary source of effective series resistance. Placing the diode near IC terminals would help to reduce series resistance. In reality, however, the diode is often a distance from the IC terminals due to area constraints. Because the effective series resistance may be substantial in an IC such as a microprocessor, it would be desirable to include its effects in the temperature calculation.
In addition, as the trend in IC design continues toward smaller chips, the power density increases and becomes less uniform. This causes the thermal gradients across the die to become greater. As a result, even though previous ICs were able to suffice with a single thermal diode at a single location on the die, future ICs may require multiple thermal diodes in order to map out the thermal profile in better detail.
Because IC terminals are at a premium on chips, the addition of thermal diodes beyond the first one may be cost prohibitive. Therefore, it would be desirable for a circuit to allow for the placement of multiple thermal diodes on the die while minimizing the number of IC terminals used.